This invention relates generally to thermoelectric elements. Thermoelectric elements are provided together with structures and devices incorporating such elements.
Thermoelectric energy converters (TECs) are devices which use thermoelectric materials for energy conversion. Thermoelectric materials exhibit the thermoelectric effect whereby a thermal gradient is generated in the material in response to an applied voltage, or a voltage is generated across the material on application of a thermal gradient. TECs can thus be used to derive electrical power from a thermal gradient or to generate a thermal gradient for heating or cooling purposes from an applied voltage.
The efficiency of a TEC is expressed by the dimensionless thermoelectric figure of merit ZT. This can be described by the expression ZT=σS2/κT, where σ is the electrical conductivity, κ is the thermal conductivity, T is the absolute temperature, and S is the Seebeck coefficient. It is generally assumed that a ZT value of 3 to 4 is required for economic power generation on a large scale. In bulk materials, σ, S and κ are interdependent and a ZT of greater than 1 is known to be difficult to achieve. Recent progress using nano-structured materials suggests that these parameters can be tuned separately. Nano-composites and superlattices combining metallic and semiconducting materials show a higher power factor σS2 than their respective bulk materials. These heterostructures combining different materials offer improved performance due to the energy filtering effect of electronic barriers. This effect, whereby an energy barrier is formed due to the particular combination of materials in the heterostructure, is discussed, for example, in “Improved Thermoelectric Power Factor in Metal-based Superlattices”, Vashaee et al., Physical Review Letters, Vol. 92, No. 10, 2004.
Current state-of-the-art nano-composite based TECs achieve ZT values of 2 at best, and the maximum value is achieved only at very high temperatures or in a very narrow temperature range. Studies on one-dimensional systems such as semiconducting nanowires have shown that this geometry can enhance ZT by lowering the thermal conductivity κ. However, a general problem in heterostructures is the presence of interface states that tend to lower electrical conductance due to uncontrolled charge carrier scattering. Fabrication of suitable structures is also difficult and expensive. While heterostructures based on planar and one-dimensional nanostructures offer more precise control of the electronic barriers than nano-particle based composites, these require even more expensive fabrication techniques. In the case of nanowire arrays, for instance, the volume density poses an additional problem.
US Patent Application Publication US2008/0276979 discloses a strain superlattice nanowire. Quantum dots are arranged in anti-phase on opposite surfaces of a nanoribbon, whereby the dots on opposite surfaces are not positioned directly opposite one another but are at offset positions along the length of the ribbon. Lattice mismatch between the dot and ribbon materials induces intermittent surface strain modulation, producing a periodic variation in the band gap of the ribbon and hence a miniband structure for increasing the Seebeck coefficient.